I've been there- trust me. I've been claustrophobic since I was nine, and I've developed ways to get through crowded places without completely having a meltdown.

Take lots of deep breaths and try to look at the floor (unless you are crossing roads or anything like that).

Take off your coat/sweater if you find yourself overheating.

Look for gaps in the crowd, and try to dodge as much human contact as possible.

Step into a nearby shop/ bathroom/ any room without a lot of people in until the crowd dies down.